1. Field of the Invention
The present invention relates to a constant current control circuit for a power converter, and more particularly, to a constant current control circuit for a power converter capable of improving line regulation.
2. Description of the Prior Art
Please refer to FIG. 1. FIG. 1 is a diagram showing a power converter of the prior art. As shown in FIG. 1, the power converter 100 of the prior art comprises a power switch 110 coupled to a primary winding P1 of the power converter 100, and a current sensing resistor Rs coupled to the power switch 110. In order to control the power converter 100 to operate under a constant current mode, a control circuit 120 of the power converter 100 of the prior art is configured to control on and off states of the power switch 110 according to a current sensing voltage Vcs of the current sensing resistor Rs, so as to keep a maximum level of a current Is flowing through the power switch 110 constant, such that an average output current Io of the power converter 100 is kept within a predetermined range.
Please refer to FIG. 2 and FIG. 3, and refer to FIG. 1 as well. FIG. 2 is a diagram showing waveforms of related current signals when an input voltage of the power converter is at a lower level. FIG. 3 is a diagram showing waveforms of related current signals when the input voltage of the power converter is at a higher level. As shown in FIG. 2, when the input voltage Vin of the power converter 100 is at a lower level, the current Is, which flows through the power switch 110, may rise up to a predetermined current level Ip slower, thus a turn-on time length Ton of the power switch 110 is longer. As shown in FIG. 3, when the input voltage Vin of the power converter 100 is at a higher level, the current Is, which flows through the power switch 110, may rise up to the predetermined current level Ip faster, thus the turn-on time length Ton of the power switch 110 is shorter. In addition, when the power switch 110 is turned off, an output current Id at an output end N1 of the power converter 100 drops in a constant speed. Therefore, a turn-off time length Toff of the power switch is a fixed value.
However, according to the above arrangement, the average output current Io outputted at a load end N2 of the power converter 100 may change. For example, as shown in FIG. 2, when the input voltage Vin of the power converter 100 is at the lower level, the power converter 100 outputs the output current Id at the output end N1 three times during a predetermined period T; and as shown in FIG. 3, when the input voltage Vin of the power converter 100 is at the higher level, the power converter 100 outputs the output current Id at the output end N1 four times during the predetermined period T. In other words, when the input voltage yin of the power converter 100 is higher, the average output current Io at the load end N2 is higher as well. Therefore, as shown in FIG. 4, when the input voltage Vin of the power converter 100 is changed, the average output current Io of the power converter 100 has larger variation, that is to say, the power converter 100 of the prior art has bad line regulation under the constant current mode.